Variable impedance wave guide matching transformer



H. c. HEY 2,527,817

VARIABLE IMPEDANCE WAVE GUIDE MATCHING TRANSFORMER Oct. 31, 1950 2 Sheets-Sheet 1 Filed Oct. 10, 1945 INVENTOR HGHE) ATTOR EV MOVEAELE PLATE ME 510 came I r Zn 2 "*(I/ J r Oct. 31, 1950 H. c. HEY 2,527,817

I VARIABLE IMPEDANCE WAVE GUIDE MATCHING TRANSFORMER Filed Oct. 10, 1945 i2 Sheets- Sheet 2 FIG. 4 38 VARIABLE WIDTH TRANSFORMER VARIABLE a SCIIVNEIP LENGTH H6: 5 v FIG. 6/!

7x9 FMED 60,05 DIRECTION OF POWER FLOW MOVES 4 21 4;

: DIPOLE P05! news rmso Pu TE F/G. 6B

DIRECTION STEP TRANSFORMER Ag arrows FLOW SECTION I \{la/ mEms /v FIG. 5A 3 (I) VAR/ABLE cums mpsomvcs \6IPOLE POSITIONS STEP TRANSFORMER (z) TRANSFORMER IMPEDANCE IN l/EN TOP (3) FIXED cums IMPEDANCE C HE'y :14 pm 51;: 60105 WIDTH (22") ATTOR EV Patented Oct. 31, 1950 VARIABLE IMPEDANCE WAVE GUIDE MATCHING TRANSFORMER Herman 0. Hey, Fort Lee, N. J., assignor to Bell I Telephone Laboratories, Incorporated, New

York, N. Y., a corporation of New York Application October 10, 1945, Serial No. 621,580

5 Claims.

. 1 1 n V This invention relates to impedance transformers for smoothly matching the impedance of wave guides to each other.

A principal object of the invention is to provide a variable impedance transformer for matching the impedance of a wave guide of fixed crosssection to a wave guide having a Varying crosssectional dimension. Another object of the invention is to provide a'variable impedance transformer for matching a wave guide of fixed cross-section to a Wave guide formed of movable parts, which produce a cross-section having avariable dimension.

A feature of the invention is a cut-out step formation in a rectangular wave guide of variable cross-section, for matching it to a guide of fixed rectangular cross-section.

Referring to the figures of the drawing:

Fig. 1 shows an antenna system and wave guide feeds, with which the variable impedance matching transformer may be used;

Fig. 2 shows a view of the variable wave guide, contractedjfwith the impedance transformer;

Fig. 3 shows a corresponding view of the variable wave guide expanded;

' Fig. 4 shows an alternative form of the variable impedance transformer;

Fig. 5 is a schematic and explanatory View of the wave guides and the impedance matching transformer; and 7 Figs. 6A and 6B show the wave fronts in the variable guide as the wide dimension varies.

Wave guide impedance matching transformers for connecting wave guides of different crosssections have been disclosed heretofore, e. g.,

in the United States Patent to G. C. Southworth No. 2,106,769, issued February 1, 1938.

Certain antenna systems are characterized by a wave guide feed having a variable dimension in its cross-section for producing electrical scanning by phase. velocity variation. One of the problems encountered in the successful operation of such a system is to provide a simple means for connecting a wave guide of fixed cross-sectional dimensions to the variable guide. A principal objective in the solution of this problem is to avoid mechanical complexity, additional moving parts, etc.

In practice, this expedient was found to be impractical and a simpler and more efiicient coupling had to be devised.

Heretofore, a coupling characterized by a fixed taper feed, and a mechanical device which always kept the taper centered with the fa dimension, has been used. a

In accordance with the invention, a variable impedance matching transformer between a rectangular guide of fixed cross-section and a similar guide having a variable (a) dimension in its cross-section, has been achieved by the simple expedient of cutting a step in the latter.

This does away with'the center in feed and with all extra mechanical and moving parts required-in the center in feed and at the same time provides a better impedance match over the band of the variable (a) scanner.

In one embodiment of the invention, the step is cut into a fixed channel portion of the variable guide, the depth of the step being determined by Z1=qimpedance of the variable guide at minimum guide cross-section,

ZT=impedance of the transformer at minimum guide cross-section,

Z2=impedance of the fixed guide,

and its longitudinal extent 2 being determined by a=wide dimension (variable) b=narr0w dimension (fixed) In one practical application of the invention, the impedance matching transformer and the associated matched wave guides have been used in a radar antenna, utilizing a linear array of 250 spaced dipoles for. radiating a very narrow beam of radiation, which scans electrically a sector 30 degrees wide.

As is Well known, the radiation pattern of a single horizontal dipole remote from the ground may be represented by a solid of revolution formed by ,a circle tangent to the dipole and rotated about the dipole axis. As the number of dipoles in a linear array is increased, the pattern becomes narrow and the circle of revolution degenerates into a long narrow loop of revolution. Thus, the sharpness of the directional pattern may be increased with increasing length of the array. When the array comprises 250 halfwave dipoles fed in phase with one another, a

very narrow beam of radiation may be produced 1 confined to a direction perpendicular to the length of the antenna array. The radiated energy may be thought of as following radial paths on a plane surface normal to the antenna. A ua the beam W h is p x mat ly de ree.

Since mechanical means for scanning are impracticable in this case, the beam may be l cte t Sea elect ica y bysh tine h phase of the currents in the respective dipoles, and this may be accomplished ,by varying the wide dimension of movable wave guide feed into which the Dick-1 D Pro e o the i o p ec Mechanical construction of variable wave guide and impedance transformer Referring specifically to Fig. 1, an electrical scanning system for radars, used preferably on airplanes, and more fully disclosed in the United States applieatiens of N. Nebel, Serial No. 01 fi ed ul 2 .1 ne Paten Q- 2. 564, issued Au ust .5, 1950, and of A K. Schenck, Serial No. 607,055 filed July 25, 1945, is shown.

'In Fig. 1, a main wave-guide 53 serves to connect a fixed frequency transceiver 5| and the variable wave-guide feed 33 for energizing the antenna array of dipoles 32. The array and feed is similar to the velocity variation type disclosed in U. S. application Serial No. 496,325 of C. B. H. Feldman, filed July 2'7, 1943. Dipoles 32 in the array are uniformly distributed along the length of the variable waveguide 33, with their probes extending into the interior thereof.

The variable wave-guide 3s ,comprises'a hollow pipe of rectangular cross-section formed of L- shaped plates 33, 35 abutting each other to form a cross-section, having its a dimension or wide dimension variable. The energy from the transceiver 5! is fed into the variable guide 33 at each end thereof by means of a U-shaped elbow pipe section 3|, of fixed rectangular cross-section.

The direction of flow of energy into either end of the wave-guide 33 is regulated by means of the radio frequency switch 98 which comprises an aluminum vane 99 attached to a core which is positioned in the field of an electromagnet, as disclosed in the aforementioned Nebel application.

The portion of the wave-guide 33, to which is attached the dipole array, is of variable (a) vertical dimension, the movable L-shaped plate member 35 being positioned to ride on plate 34 by means of the knob 36 on the eccentric cam 54. The cam 54 is rigidly fixed to the drive shaft 66, which is attached to the drive motor 47 electromagnetic waves traveling at an angle to each other and successively reflected from the side walls thereof.

The TEo,1 mode in a rectangular guide may be considered as the resultant arising from the superposition of two plane waves moving in crossdirectionsat an angle depending upon the frequency of the waves and the size of the guide. Usually, the plane, electric field components traveling at an angle across the wide or "a.

dimension of the guides are considered. This r. Cu

is illustrated in Figs. 6A and 6B where the wavefronts of the plane waves are shown traveling in zig-zag formation through the guide. The separation of the resultant crests and troughs, determines the wavelength in the guide. As the guide is made smaller in its widedimens qn, these crests and troughs become 16 Widely spaced and hence the wavelength is increased.

As the wavelength in the guide changes, the antenna probes which are in a fixed position appear at different instantaneous phase positions in the passing cycles of the Waves traveling through the guide. Therefore, as the Wide dimension becomes smaller the fixed probes become closer together as measured in wavelengths. This results in a uniform change in phase of the currents in the dipoles from one end of the array to the other. The points at which the electric field intensities reinforce each other in space is consequently altered so that instead of the elements of radiation forming a sheet perpendicular to the array, the elements now all make an angle 0 with the perpendicular. When the currents are traveling from right to left, the beam is defiected to the right and conversely, thereby producing scanning electrically.

Since there is a limit to which the wide dimension of the guide may be varied and still propagate the TE0,1 wave, there is a limit to the amount of electrical scanning possible. The limit of motion has been restricted to a -1.20-inch maximum and 0.665-inch minimum (inner dimension) which develops approximately a -degree lead in respective dipole currents in the direction of energy flow. A resultant beam deflection of 30 degrees is obtained for a single cycle of motion of the variable guide.

However, by feeding power into the antenna wave guide from the otherend, the currents can be made to lead from theopposite .end so that a scan angle of 30 degrees onthe opposite side of 0 degree (straight ahead) is obtained. This gives a total scan angle of 60 degrees in the forward direction.

In Figs. 2 and 3, the variable wave guide 33 which feeds the dipoles of the antenna array forms part of an aluminum channel 38 provided length as shown in 'Fig. 2. A movable L'-shaped aluminum plate 35 with a similar raised parallel section located two-thirds of the distance from the bottom is placed along the inner side of the channel. 7

The parallel raised sections thereby formtwo opposite relatively movable outer walls of a variable rectangular wave guide 33. The L-shaped plates 34, 35 which. together constitute the variable guide are held together by means of springs and rollers at equally spaced points therealong.

Fig. 3, which is identical in structural details to Fig. 2, shows the variable wave guide 33 with the rectangular, cross-section expanded due to the increase in the a dimension. V .Thef'rate of the vertical' m'otion of L-shaped plate 35 is such that a uniform rate of scanof the radiation beam is attained; v I

In order to obtain'approximately equal power of pick-up from the successive dipoles, the first one, which is exposed to the maximum power,

should be inserted alesseramount than succeedirigones. Preferably, the succeeding dipoles should be inserted to greater depths at approximately an exponential rate. However, to permit a similar distribution of energy pick-up from both ends, a compromise may be effected, wherein the increase of penetration progresses from each end to a maximum at the middle.

A variable, impedance matching transformer 23 is formed in the movable guide 33 at the transition points from the fixed to the movable wave guide. The transformer comprises a step cut into the L-shaped plate 34 on the movable guide 33, as shown in Figs. 2 and-3, and sche-.

matically in Figs. 5 and 6B, the depth and longitudinal dimension thereof being determined by Equations 1 and 2, supra. The step sets up an impedance transformer section ZT between Z2 of the fixed guide and Za of the variable guide.

In the operation of the transformer, as illustrated in Fig. 5,

Z'T=\/ZaZ2 1 (4) where Zr and Za represent the impedances of the transformer section and variable guide, respectively, both varying as plate 35 is moved along plate 34.

The improvement in performance resulting from the variable transformer section is shown.

in Fig. 5A. Curve 2 shows the transformer impedance as a function of the a dimension of the variable guide. As the a dimension varies, the transformer provides a better match between the impedance of the-variable guide, illustrated by steep curve (I) and the constant impedance value, curve (3), characteristic of the fixed guide. In general, the interposition of the impedance transformer section serves to limit the standing wave ratio to 1.25 or less.

The transformer action represents a compromise match over the variable a region. Its interposition between the fixed and variable wave guide in a practical radar embodiment, contributed to reliable performance and successful operation of the aforementioned scanning system.

pedance transformerthe spacing therebetween being. variable by moving plate 35" on which block H is fastened, along the base 34' of channel 38;

6. Block' IU 'isfixedto the base. The length of the" blocks and their relative spacing apart is' determined from Equations 2 and 1, supra.

In one physical embodiment of the form shown in Fig. 4, the transformer was designed on the following numerical basis: 1 I Z1=Impedance of. variable guide at minimum guideiwidth, 'a=.6'70". I Z2"=Impedance of fixed feedguide, a=1.125".. zfrlmpedance of transformer at minimum and the transformer a-r dimension is calculated and determined from Z'r. Similarly, the length of the transformer AT/4 is made equal to the geometric mean between the transformer wavelength at minimum and maximum transformer widths. For the linear dipole antenna array these transformer constants were determined to Whereas the transformer has been disclosed as applied to a linear array of dipoles, it should be understood that the antenna may be of any suitable type known to the microwave art, such as disclosed, for example, in the aforementioned Feldman application or equivalents thereof.

What is claimed is:

1. In combination, a wave guide of fixed crosssection having a characteristic impedance, a wave guide of continually variable cross-section having a continually variable characteristic impedance, said variable wave guide having an end contiguous an end of said fixed guide with a portion of the end faces thereof in sliding contact, means for matching said wave guides comprising a step cut in said contiguous end of said variable guide, said step having a variable impedance at all times intermediate said impedance of the fixed guide and said variable impedance of the variable guide.

2. In combination, a wave guide of fixed crosssection, a wave guide of continually variable cross-section comprising separable parts coextensive in length, and an impedance transformer for matching said guides comprising a third wave guide connected between said fixed guide and said variable guide, said third guide abutting said fixed guide and adapted to vary uniformly in crosssection with said variable guide, the impedance of said impedance transformer at minimum guide cross-section being the mean of said variable guide impedance at minimum guide cross-section and the impedance of said fixed guide.

3. In a high frequency electrical wave transmission system comprising a rectangular wave guide of fixed cross-section having a and "b cross-sectional dimensions and a rectangular wave guide of continually variable cross-section having a and b cross-sectional dimensions, dimension a being variable, means for coupling and matching said guides comprising a wave guide section connected between said fixed and said variable guides as an integral extension of said variable wave guide, said section having an a dimension varying concurrently with said "a dimension of said variable guide by virtue of said integral connection and remaining at all times intermediate said last-named dimension and said fixed a dimension.

4. In combination, a wave guide of fixed rectangular cross-section, a wave guide of continually variable rectangular cross-section having a var b e dimension in the w e di ection hereoi, and means for coupling said guides comprising a rectangular w ve gu de section conn ct d as n integral extensionfof said variable wave guide, the end of a d wa e u dev ec o o o t d variable guide and adjacent said fiXedguide having a portion of the end face thereof in sliding contact with the end face of said fixed guide, said guide section having a variable cross-section with the wide dimension thereof being intermediate the wide dimensions of said fixed guide and said variable guide, the Wide dimension thereof bein variable concurrently with said wide dimension of said variable wave guide, said guide section proportioned to have a length substantially equal to one fourth of the mean wavelength of the waves in said section at a predetermined operating frequency.

;5. The structure of claim 4 wherein the wide dimension of said guide section is proportioned to give said guide section a characteristic impedance at its minimum cross-section equal the square root of the product of the impedance of the variable guide at minimum guide cross-section and the impedance of said fixed guide, and wherein the length of said guide section is equal the square root ofone quarter of the product of the wavelengths in the variable guide at minimum cross-section and at maximum cross-section.

HERMAN C. HEY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,106,768 Southworth Feb. 1, 1938 2,106,769 Southworth Feb. 1, 1938 2,232,179 King Feb. 18, 1941 2,403,289 Korman July 2, 1946 2,423,396 Linder July 1, .1947 2,427,098 Keizer Sept. :9, 1947 2,433,368 Johnson Dec. 30, 19.47 2,480,208 Alvarez Aug. 30-, 1949 

